philadelphus vs syringa

philadelphus and syringa both are nouns.

Word	Noun	Adjective	Verb	Adverb
philadelphus	Yes	No	No	No
syringa	Yes	No	No	No

As nouns, syringa is a hyponym of philadelphus; that is, syringa is a word with a more specific, narrower meaning than philadelphus:

philadelphus: any of various chiefly deciduous ornamental shrubs of the genus Philadelphus having white sweet-scented flowers, single or in clusters; widely grown in temperate regions
syringa: large hardy shrub with showy and strongly fragrant creamy-white flowers in short terminal racemes

Other hyponyms of philadelphus include mock orange, Philadelphus coronarius.

philadelphus (noun)	syringa (noun)
any of various chiefly deciduous ornamental shrubs of the genus Philadelphus having white sweet-scented flowers, single or in clusters; widely grown in temperate regions	large hardy shrub with showy and strongly fragrant creamy-white flowers in short terminal racemes